What are the odds of the last three digits of my post number being the same, with a crazy variable?

Question

I'm really new to this stack so I appreciate any tips on how to better phrase my question.

There's a website called 4chan.org, an imageboard that has up to 22 million monthly visitors across 71 different boards.

On a certain board, I made a post with the post number 185,638,777. In the same thread, I added another post with the post number 185,639,111

Crazy variable: I ended up replying to my own post (see attached image). I don't how to quantify the probability of that, or even put it into words, but if anyone can solve it be my guest.

Here is the post in question

The questions I have are:

1. What are the odds of the last 3 digits of my post number being the same
2. What are the odds of the last 3 digits of my post being the same, and me replying to the first post?

Answer 1

The probability is $\frac{10}{1000}=\frac1{100}$ For another one to also have the last three digits the same, you square the probability: $(\frac1{100})^2=\frac1{10000}$